Wafer planarization using a uniform layer of material and method and apparatus for forming uniform layer of material used in semiconductor processing

ABSTRACT

In connection with wafer planarization, an apparatus for forming a layer of material having a substantially uniform thickness and substantially parallel first and second major surfaces includes a pair of pressing elements and a stop. Each of the pair of pressing elements has a flat pressing surface. The pressing surfaces are opposed to one another and operable to compress a quantity of the material therebetween. The stop is positioned at least partially between the pressing surfaces and has a thickness substantially equal to the desired uniform thickness of the layer. The stop is positioned to establish a spacing between the flat pressing surfaces that is substantially equal to the thickness of the stop and thereby to the desired uniform thickness of the layer when the pressing elements engage the stop. As a result, engagement of the stop by the pressing surfaces during pressing of the material forms a layer of the material of substantially uniform thickness with substantially parallel major surfaces formed by the flat pressing surfaces. The layer is then used in semiconductor processing to provide a flat surface on a layer of a substrate assembly, thereby enhancing the planarization of the substrate assembly.

BACKGROUND OF THE INVENTION

[0001] This invention relates to methods and apparatus for forming auniform layer of material for use in connection with manufacturing asubstrate assembly during semiconductor processing, and also the layeritself. The invention also relates to a method of planarizing asemiconductor wafer. As used herein, “substrate” refers to the lowestlayer of semiconductor material in a semiconductor wafer, and “substrateassembly” refers to a substrate having at least one additional layerwith structures formed thereon.

[0002] “Semiconductor flat” refers to a surface of the substrateassembly having a precision flat surface within desired tolerances. Asignificant aspect of semiconductor processing is planarization, i.e.,ensuring that the semiconductor flat and other layers are planar withina predetermined specification.

[0003] Production methods for semiconductors are known. A particularclass of methods involves: etching or otherwise forming desired channelsor trenches in a substrate assembly surface, applying a dielectric epoxylayer which fills the trenches over the substrate assembly surface,using an apparatus to press the substrate assembly having the epoxylayer to achieve desired surface characteristics (e.g., flatness) on theepoxy layer, and then removing the pressed substrate assembly from theapparatus for further processing. The epoxy may be of a type which iscured with ultraviolet radiation.

[0004] Removing the pressed substrate assembly from the apparatus isdifficult, however, because the epoxy begins bonding with the pressingsurface. Therefore, according to some methods, the epoxy layer is firstcovered with a layer of a cover material before the pressing takesplace. The cover material is selected to allow easy removal/release ofthe pressed substrate from the apparatus.

[0005] In addition, the cover or release member must be transparent tothe ultraviolet radiation if an epoxy of the type cured by ultravioletradiation is used to cure the epoxy layer beneath the cover material. Ithas been previously determined that fluorinated ethylene-propylene (FEP)can be used as the cover material. Some types of FEP are transparent toultraviolet radiation, and thus do not affect the epoxy curing byultraviolet radiation passing through the cover.

[0006] The cover material is placed over the epoxy layer before thesubstrate assembly is pressed, and thus the cover material surfacecharacteristics are transferred to the substrate assembly surface. Ifthe cover material is a uniform layer, which is defined as a layerhaving parallel major (top and bottom) surfaces that are planar, withinpredetermined tolerances, the pressing action applied through the covermaterial will be uniformly transferred to the epoxy layer as desired. Asone result, if the cover material is a uniform layer, the substrateassembly surface can be formed to the same flatness as the pressingsurface.

[0007] In practice, achieving a sufficiently uniform layer of a covermaterial such as of FEP has not been achieved utilizing knowntechniques. Because of the nature of FEP material and the desiredthickness of a typical cover (about 0.020 in. thick), the dimensions ofa FEP cover are difficult to control. For example, in one approach whereultraviolet transmissive FEP has been heated to a temperature below itsmelting point and pressed between two optical flats during pressing, themajor surfaces of the resulting FEP layer end up significantly skewed orout of parallel from one another. As used herein, optical flats aredefined as precision pressing surfaces, e.g., surfaces that are flat towithin one quarter of a wavelength of light.

[0008] The temperature range for processing the FEP is very narrow. Anacceptable temperature is slightly below the melting glass flowtransition point, which allows the FEP material to acquire the surfacesmoothness characteristics of the optical flats. Since high pressuresare required to make the FEP surface conform to the optical flatssurfaces, at temperatures below the glass transition point (i.e., in theplastic state), maintaining the material at a consistent thickness isvery difficult. This difficulty is due to the uncontrolled movement ofFEP material from the higher pressure zones to the lower pressure zonesat the perimeter of the pressing mechanism. Consequently, the thicknessof the layer is no longer satisfactorily uniform.

[0009] When used as a cover layer, this non-uniformity in thicknesscaused variations in the thickness of the epoxy layer. Consequently,during subsequent semiconductor wafer processing, involving etchingthrough the epoxy layer, undesirable non-uniform etching would occurbecause thinner portions of the epoxy layer would be etched throughfirst. For example, FEP sheets exhibiting these problems had majorsurfaces which were flat to within about 30-35 angstroms, but which wereonly parallel to one another within +0.010 in., have been obtained usingknown processes.

[0010] Accordingly, it would be desirable to provide a method andapparatus by which FEP and other materials used as cover layers on asubstrate assembly could be produced within desired uniform layerspecifications.

SUMMARY

[0011] Wafer planarization is enhanced utilizing a layer of materialhaving a substantially uniform thickness and substantially parallelfirst and second major surfaces. The layer is used in producing a flaton or planarizing a substrate assembly.

[0012] In one embodiment, an apparatus having a substantially uniformthickness and substantially parallel first and second major surfacesincludes a pair of pressing elements and a stop. The layer of materialformed by the apparatus used in producing a flat on semiconductors. Eachof the pair of pressing elements has a flat pressing surface. Thepressing surfaces are opposed to one another and operable to compress aquantity of the material therebetween. The stop is positioned at leastpartially between the pressing surfaces and has a thicknesssubstantially equal to the desired uniform thickness of the layer. Thestop is positioned to establish a spacing between the flat pressingsurfaces that is substantially equal to the thickness of the stop andthereby to the desired uniform thickness of the layer when the pressingelements engage the stop. As a result, engagement of the stop by thepressing surfaces during pressing of the material forms a layer of thematerial of substantially uniform thickness with substantially parallelmajor surfaces formed by the flat pressing surfaces.

[0013] The apparatus can also include a heater that heats the materialto a temperature where it flows without melting. Further, the apparatuscan include a compression force applicator to move one or both of thepressing surfaces. The compression force applicator can include aplurality of biasing elements.

[0014] The pressing surfaces can be optical flats. The shim can have aplurality of projections extending inwardly from the border portion withoverflow material recesses positioned between the projections. Theprojections can be of a triangular shape.

[0015] In a specific example, the first and second major surfaces of thelayer are each within 100 angstroms of being flat. Preferably, in thisexample, the first and major second surfaces of the layer are at leastwithin 0.000005 in. of being parallel to one another. In this example, astop portion of the shim is about 0.020 in. thick. The cover layer mayalso be transparent to ultraviolet radiation.

[0016] According to an exemplary method, a layer is formed by heatingmaterial and pressing the material between first and second flatpressing surfaces. A stop is disposed between the first and secondpressing surfaces to limit the extent to which the first and secondpressing surfaces approach one another during pressing to thereby form alayer of substantially uniform thickness having first and second majorsurfaces with the first and major second surfaces being formed by theflat pressing surfaces. Thereafter, one of the first and major secondsurfaces of the formed layer may be applied to a flat surface of asubstrate assembly. In this approach, the heating step may includeheating the material until the material transitions to a plastic statewithout melting.

[0017] The formed layer may be applied, for example, over an epoxy layerof a substrate assembly. The assembly may then be pressed by precisionoptical flats with the flatness of the optical flats being transferredto the epoxy layer through the formed layer. The formed layer in thiscase prevents the epoxy layer from adhering to the pressing apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 is a side view of an apparatus for achieving a uniformthickness of a material to be applied to a substrate.

[0019]FIG. 2 is a top view of an upper lid of the apparatus of FIG. 1.

[0020]FIG. 3 is a bottom view of a lower lid of the apparatus of FIG. 1.

[0021]FIG. 4 is a top view of the shim of the apparatus of FIG. 1.

[0022]FIG. 5 is a side sectional view of the shim of FIG. 4 along theline V-V.

[0023]FIG. 6 is a magnified view of an edge portion of the shimsectional view of FIG. 5 showing a tooth portion.

[0024]FIG. 7 is a side sectional view of the shim of FIG. 4 along theline VII-VII and corresponding to FIG. 6, but showing an open region.

[0025]FIG. 8 is a side sectional view of an edge portion of theapparatus showing the upper optical flat beginning to press againstmaterial applied on the lower optical flat with the shim between theupper and lower optical flats, while being heated in an oven.

[0026]FIG. 9 is a side sectional view of a portion of the apparatus ofFIG. 8 showing the apparatus after pressing is complete with the upperand lower optical flats in contact with the shim and the material withinthe shim pressed to a uniform thickness.

[0027]FIG. 10 is a graph of time-temperature profiles showing thetemperatures of a heater element, an oven air temperature and arepresentative FEP material being pressed during a heating process.

[0028]FIG. 11 is a schematic side view of a substrate assembly with acover layer applied over an epoxy layer.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0029]FIG. 1 shows one form of a press assembly 100 for achieving adesired uniform layer of a material to be applied on a substrateassembly during manufacture. The uniform layer is used in producing aflat on a semiconductor. The assembly 100 includes an upper lid 102, alower lid 104, an upper optical flat 112, a lower optical flat 114 and astop which limits the extent to which flats 112, 114 approach oneanother and which may take the form of a shim 118. In the illustratedembodiment, these components each have a generally circular periphery,and are coaxially aligned with each other. For clarity, the upper lid102 and upper optical flat 112 are shown spaced from the shim 118, loweroptical flat 114 and lower lid 104.

[0030] During operation of the assembly 100, the upper optical flat 112and the lower optical flat 114 serve as pressing elements that arepressed together under predetermined heating conditions against the shim118, thereby pressing material applied on the lower optical flat 114within the shim 118 to a uniform thickness. As shown in FIG. 1, a lowerside of the shim 118 contacts an upper side or pressing surface of thelower optical flat 114. A lower side of the lower optical flat 114contacts a supporting surface 108 of the lower lid 104.

[0031] The shim 118 may be annular or ring-shaped with projections thatextend inwardly and space the flats apart to a desired uniform distancewhen engaged by the flats. The projections may comprise a plurality ofspaced apart fingers. In the specific form shown, the fingers comprisetooth points 134 that project inwardly at regularly spaced intervals(FIG. 4) from a border 136. Alternatively, the shim 118 may take otherforms such as being shaped as an ellipse, triangle, square, rectangle orother closed geometrical shape. The tooth points 134 do not span theentire interior of the shim 118 and thus define an open center area orvoid 144. Communicating with the open center area 144 are overflowmaterial receiving recesses, pockets or open regions 138 that liebetween adjacent tooth points 134. Material in the open center area 144is pressed to a desired thickness B, which is equal to the thickness ofthe tooth points, when the upper optical flat 112 and the lower opticalflat 114 are pressed together in a press direction A against the toothpoints 134.

[0032] As described below, excess material is pressed outward frombetween the upper optical flat 112 and the lower optical flat 114through the open regions 138. The excess material flows outward from theopen center area 144 through the open regions 138 into areas adjacentthe periphery of the first optical flat 112 and the second optical flat114.

[0033] The pressing action in the press direction A is achieved througha compression force or pressure applicator. In an illustratedembodiment, the pressing action is achieved using elongated fasteners orbolts 120 that slidably extend through apertures 122 in the upper lid102 and apertures 124 in the shim 118, and are threaded into apertures126 in the lower lid 104. Threaded ends of the bolts 120 are received inhelicoils 132 positioned within the apertures 126. The bolts 120 areeach inserted through one or more biasing elements. In the form shown,the bolts 120 are each inserted through a pair of Belleville washers128, 130 oriented in a stacked back-to-back orientation to create apressing action when the bolts 120 are tightened. The illustratedassembly 100 is preferably secured together by six such bolts 120 atequally spaced intervals, but for clarity, only two bolts 120 are shownin FIG. 1. Prior to pressing, the upper optical flat 112 may beseparated from the shim 118 by, for example, approximately {fraction(3/16)} in.

[0034] The upper optical flat 112 and the lower optical flat 114 arecylindrically shaped and each have at least one precision pressingsurface. The pressing surfaces are preferably flat to at least to within100 angstroms and more preferably flat to at least within 50 angstroms.In a specific example, these optical flats are half-wavelength flatshaving a flatness of 30-35 angstroms. The optical flats may be made of aquartz material. Although the size of the flats may vary in a specificexample, they have a diameter of approximately 9 in. and a thickness ofapproximately {fraction (11/2)} in. Thus, the upper lid 102, the lowerlid 104, the shim 118 and the bolts 120 are sized accordingly.

[0035] To prevent damage to the quartz material, the upper lid 102 andthe lower lid 104 may have an upper supporting surface 106 and a lowersupporting surface 108, respectively, with beveled ends 110. The edges116 of the upper optical flat 112 and the lower optical flat 114 arespaced outward of the beveled ends 110. As a result, the edges 116 ofthe upper optical flat 112 and lower optical flat 114 are not directlyloaded during pressing.

[0036] The upper lid 102 and the lower lid 104 may be made of a heatconducting material such as aluminum. The shim 118 may be, for example,made of stainless steel. The Belleville washers 128, 130 may also bemade of stainless steel and rated at, for example, 150 lbs.

[0037]FIG. 2 is a top view of the upper lid 102 showing its uppersurface. FIG. 2 shows the six equally spaced apertures 122 separatedfrom each other by an angle E (i.e., 60°). FIG. 2 also shows therelative positions of the upper optical surface 106 and the bevel 110 onthe lower surface of the upper lid 102.

[0038]FIG. 3 is a bottom view of the lower lid 104 showing its lowersurface. Similar to the upper lid 102, FIG. 3 shows the six equallyspaced apertures 126 separated from each other by the angle E, as wellas the uniform lower support surface 108 and the bevel 110 on the uppersurface of the lower lid 104. The apertures 126 of the lower lid 104 arefitted with helicoils 132 (not shown), as described above, for receivingthreaded ends of the bolts 120.

[0039]FIG. 4 is a top view of the illustrated shim 118 showing its uppersurface with the border portion 136 from which the inwardly projectingtooth points 134 extend. The six equally spaced apertures 124 shown inthis example extend through the border portion or reinforcing section136. Each tooth point 134 defines an acute included angle F. Althoughvariable, in the form shown, the angle F is 30°. Apexes of adjacenttooth points 134 are separated from each other by an acute tooth pointspacing angle G. In the illustrated embodiment, the angle G, although itmay be varied, is 10°, and thus there are 36 tooth points 134 total.There are also 36 open regions 138 interspersed between adjacent pairsof the tooth points 134. The major surfaces (i.e., the top and thebottom) of the teeth 134 are formed to be parallel with each otherwithin a desired tolerance. In a specific example, this is +0/−0.000005in.

[0040] The open central area of the shim, between the apexes of a pairof diametrically opposed tooth points 134, is sized large enough toresult in a uniform sheet of the desired size. For example, a circularcentral area having a diameter of 8.12 inches, between the apex of atooth and the apex of a diametrically opposed tooth, may be used toproduce a circular sheet of material having the desired uniformthickness and flatness, which is at least eight inches in diameter. Theuse of pointed teeth for the projections facilitates the flow ofmaterial past the projections and minimizes the possibility ofnon-uniformities in the sheet extending inwardly into the central areafrom the teeth. Alternatively, the sheet may be made significantlyoversized, in which case non-uniformities at the edge of the sheet maybe trimmed while still having a sheet of the desired size with thedesired uniformity.

[0041]FIG. 5 is a side sectional view of the shim 118 along the line V-Vof FIG. 4. FIG. 6 is a magnified view of a right side portion of thesectional view in region VI of the shim 118 of FIG. 5. FIG. 6 shows theextent by which the tooth points 134 extend inwardly from the borderportion 136. As also shown in FIG. 6, the border portion 136 has athickness H that is substantially greater than the thickness B of thetooth points 134 extending inwardly from the border portion 136.

[0042]FIG. 7 is a sectional view of the shim 118 along the line VII-VIIof FIG. 4 on a scale comparable to FIG. 6. FIG. 7 shows the extent ofthe open regions 138 between adjacent tooth points 134, as well as theadjacent tooth point 134′ in the counterclockwise direction.

[0043]FIG. 8 is a partial side view of a right end of the upper opticalflat 112, the lower optical flat 114 and the shim 118. The portion ofthe shim 118 shown in FIG. 8 is the same as in FIG. 7, i.e., showing oneof the open regions 138 and the adjacent tooth point 134′. In FIG. 8, alayer 142 of cover material has been deposited on the lower optical flat114 and over the tooth points 134 of the shim 118, and the upper opticalflat 112 and the lower optical flat 114 are being pressed together inthe direction A, while being heated in an oven 300. As shown in FIG. 8,the layer 142 has an initial thickness C that is about two times thickerthan the desired layer thickness B.

[0044]FIG. 9 is a view similar to FIG. 8, but showing the configurationafter the upper optical flat 112 and the lower optical flat 114 havebeen pressed together until stopped by the shim 118. As shown in FIG. 9,the layer 142 has been pressed to the thickness B uniformly, and excessmaterial has been forced out from between the upper optical flat 112 andthe lower optical flat 114 along the path D through the open regions138.

[0045] Assume the layer 142 is to be of FEP, and the desired thickness Bof the layer 142 is 0.020 in. To manufacture such a layer, one specificapproach is as follows:

[0046] (1) the layer 142 is initially deposited on the lower opticalflat 114 within the open center area 144 of the shim 118 to a levelabout twice the desired thickness B (i.e., the starting thickness of theFEP may be about 0.040 in.);

[0047] (2) the assembly 100 is heated in an oven to cause the layer 142to flow, but is maintained below the melting point of FEP;

[0048] (3) a spring force in the case applied by the Belleville washers28, 30, press the upper optical flat 112 and the lower optical flat 114together, in a controlled manner;

[0049] (4) excess FEP passes outward from between the upper optical flat112 and the lower optical flat 114 and into the open regions 138;

[0050] (5) after the desired thickness B is reached, i.e., when theupper optical flat 112 bears against the shim 118, the assembly 100 isallowed to cool;

[0051] (6) the excess FEP is then removed;

[0052] (7) the bolts 120 are loosened and the upper optical flat 112 andthe shim 118 are raised; and

[0053] (8) the layer 142, which is a uniform layer having a thickness B,is removed from the lower optical flat 114.

[0054] Alternatively, only the pressing surfaces, the shim 118 and thelayer 142 need to be heated to cause the layer 142 to flow.

[0055] The raw FEP is typically provided in sheets which are normally0.04 in. thick. These sheets are typically formed using rollers and havesignificant thickness variations. Also, defects may exist in thesesheets, such as bubbles. Typically, the raw material sheets are visuallyscreened, and portions having bubbles or other significant defects thatare likely to show up in the finished layer are discarded. However,minor bubbles or defects in the raw material near the expected edges ofthe finished layer may be allowed to remain as they disappear duringpressing and flowing process of making the finished layer.

[0056]FIG. 10 is one example of a time-temperature profile of varioustemperatures in a pressing process in which FEP is used as the layer142. The curve 150 shows the temperature of a heating element within theoven. The curve 152 shows the air temperature within the oven. The twocurves 154 represent the temperature of the FEP as measured bythermocouples 156, 158 and 160 at the periphery, center, and halfwaybetween the periphery and the center, respectively, of the lower opticalflat 114 (FIG. 1).

[0057] The melting point of the specific FEP of this example is 270 C.It is desirable to heat the FEP until it transitions to a plastic stateand begins to flow, but does not melt. At point a, following a soak ofapproximately 12 hours, the temperature of the layer 142 is stabilizedat about 223 C. An extended soak period is used to prevent thepossibility of overheating the layer 142 beyond the melting point. It isalso desirable to heat the upper optical flat 112 and the lower opticalflat 114 evenly, i.e., until the temperatures of the peripheries and thecenters of the optical flats are within ½ to 1 C of each other.

[0058] After point a, the temperature of the oven is raised, as shown inthe curves 150 and 152, to increase the temperature of the layer 142slightly. Thereafter, the layer 142 reaches the temperature at which theFEP flows, and the pressing takes place until stopped by the shim 118.

[0059] In another example using PTFE as the layer 142, atime-temperature profile similar to FIG. 10 may be used. The meltingpoint of one specific PTFE is approximately 317 C, and the soaktemperature is approximately 270 C. Besides these differences, theprocess is generally similar to the process described above for thelayer 142 made of FEP. Of course, other temperature heating profiles mayalso be used.

[0060] With the pressing complete, excess material is trimmed from theassembly 100 near the peripheries of the upper optical flat 112 and thelower optical flat 114 such as with a dull knife.

[0061] The pressed uniform layer 142 is then allowed to cool, forexample, slowly to avoid thermal shock. In one process, the pressedlayer 142 is allowed to cool for approximately 6 hours. Over the courseof the cool down period, the layer 142 may shrink by 0.050 to 0.100 indiameter. After the cool down period is concluded, the pressure isreleased, and the layer 142 is complete. The cover layer may be removedand used in subsequent semiconductor processing.

[0062]FIG. 11 is a schematic side view of a substrate assembly with acover layer. As shown in FIG. 11, the uniform layer 142 that has beenpressed to uniform thickness has been applied over an epoxy layer 200 ofa substrate assembly 202 before the substrate assembly 202 issubsequently pressed and cured with ultraviolet radiation. A pressingapparatus is shown schematically, in a state separated from thesubstrate assembly 202, at 206. The epoxy layer 200 has been applied tofill trenches 204 in the substrate assembly 202.

[0063] With the layer 142 in place between the pressing apparatus 206and the epoxy layer 200, the completed substrate assembly 202 is easilyremoved from the pressing/curing assembly (if necessary, air can bedirected between the layer 142 and the pressing surface of the pressingapparatus 206 to facilitate removal). Because the layer 142 is uniform(the major surfaces are substantially flat and parallel), the precisionof the pressing surface of the pressing apparatus 206 is transferred tothe epoxy layer 200 of the substrate 202. One suitable epoxy is DEN431Novalak resin mixed with a solvent to achieve a desired consistency.

[0064] Although FEP is a preferred cover material for use as the layer142, other plastic materials that can be heated to a plastic statewithout melting can also be used, with consideration of the otherrequirements discussed above. One specific FEP is available fromMcMaster-Carr of Los Angeles, Calif. under the catalog designation85375K114.

[0065] In the methods and apparatus described above, one of the pressingsurfaces remains stationary, whereas the other of the pressing surfacesis moved. Optionally, both pressing surfaces may be moved toward eachother, as would be known to those with ordinary skill in the art.

[0066] Having illustrated and described the principles of our inventionwith reference to several preferred embodiments, it should be apparentto those of ordinary skill in the art that the invention may be modifiedin arrangement and detail without departing from such principles. Weclaim as our invention all such modifications that fall within the scopeof the following claims.

What is claimed is:
 1. In semiconductor processing, an apparatus forcompressing material into a layer of substantially uniform thickness foruse in forming a flat surface on a substrate assembly, the materialbeing of a type which is capable of being heated to a temperature wherethe material flows without melting, the apparatus comprising: first andsecond pressing elements, each pressing element having a respectivepressing surface, the pressing surfaces being opposed to one another; ashim positioned between the pressing surfaces, the shim having an opencenter area and a stop portion having a thickness equal to the desireduniform thickness of the layer; a heater that heats the material to atemperature where the material flows without melting; and a compressionforce applicator coupled to each of the pair of pressing elements, thecompression force applicator moving at least a first pressing surface ofthe pair of pressing surfaces toward the other of the pressing surfacesand against the stop portion of the shim to press a quantity of thematerial positioned between the pair of pressing surfaces and within theopen center area of the shim into the layer having the substantiallyuniform thickness.
 2. The apparatus of claim 1 wherein the compressionforce applicator moves only the first pressing surface during pressingof the material.
 3. The apparatus of claim 1 wherein the compressionforce applicator comprises a plurality of biasing elements coupled tothe first and second opposed pressing surfaces so as to bias thepressing surfaces toward one another.
 4. The apparatus of claim 1wherein the compression force applicator comprises a plurality ofelongated fasteners each interconnecting the respective pressingelements and the shim, the pressing element which includes said firstpressing surface being slidably coupled to the elongated fasteners suchthat said first pressing surface is slidable toward the other of thepressing surfaces, and a plurality of biasing spring elements coupled tosaid pressing elements to bias said first pressing surface toward theother of the pressing surf aces.
 5. An apparatus for use in forming asheet of material for use in semiconductor processing, comprising: anannular shim having a border portion and a plurality of projectionsextending inwardly from the border portion, the projections having afirst thickness, a shim open area positioned inwardly of theprojections, overflow material recesses being positioned between theprojections and communicating with the shim open area; and first andsecond pressing surfaces positioned on opposite sides of the shim, thefirst and second pressing surfaces contacting the shim at least alongthe projections to compress material on at least one of the first andsecond pressing surfaces within the shim open area to a substantiallyuniform thickness equal to the first thickness, wherein any material inexcess of the volume defined by the shim opening and the first andsecond pressing surfaces passes outwardly from the shim opening and intothe recesses.
 6. The apparatus of claim 5 wherein the first and secondpressing surfaces each comprise an optical flat.
 7. The apparatus ofclaim 6 further comprising a pressure applicator coupled to the firstand second pressing surfaces for pressing together the first and secondpressing surfaces.
 8. The apparatus of claim 5 wherein the projectionsare of a triangular shape.
 9. The apparatus of claim 5 including an ovenwhich receives at least the first and second pressing surfaces and shimfor heating the material during pressing.
 10. In semiconductorprocessing, an apparatus for forming a layer of a material having asubstantially uniform thickness and substantially parallel first andsecond major surfaces, the material being for use in producing a flat ona semiconductor, the apparatus having: a pair of pressing elements eachhaving a flat pressing surface, the pressing surfaces being opposed toone another and operable to compress a quantity of the materialtherebetween; a stop positioned at least partially between the pressingsurfaces, the stop having a thickness substantially equal to the desireduniform thickness of the layer and being positioned to establish aspacing between the flat pressing surfaces which is substantially equalto the thickness of the stop and thereby to the desired uniformthickness of the layer when the pressing elements engage the stop; andwhereby engagement of the stop by the pressing surfaces during pressingof the material forms a layer of the material for use in producing aflat on a semiconductor, the layer of material being of substantiallyuniform thickness with substantially parallel major surfaces formed bythe flat pressing surfaces.
 11. A release member for preventing asurface layer on a semiconductor substrate assembly from adhering to asurface of a press, the release member comprising: a body in the form ofa sheet having first and second major surfaces; tile first and secondmajor surfaces each being within one hundred angstroms of being flat;the first and second major surfaces being at least within fivemillionths of an inch of being parallel to one another; the body beingof a material which releases from the press surface when engaged by thepress surface; and whereby when the first major surface is pressedagainst the press surface and the second major surface is positionedagainst the surface layer on the semiconductor substrate assembly, therelease member prevents the layer on the semiconductor substrateassembly from adhering to the press surface.
 12. A release memberaccording to claim 10 wherein the first and second major surfaces are atleast within fifty angstroms of being flat.
 13. A release memberaccording to claim 11 wherein the surface layer on the semiconductorsubstrate assembly contains a layer of epoxy.
 14. A sheet of a materialused in producing a flat on a substrate assembly, the sheet of materialbeing capable of flowing without melting when heated to a temperaturebelow the melting point of the material, the sheet comprising: a bodyhaving a central region with first and second opposed major workingsurfaces; the first and second major working surfaces each being withinfifty angstroms of being flat and being at least within five millionthsof an inch of being parallel to one another.
 15. A sheet according toclaim 14 wherein the material is ultraviolet transmissive FEP.
 16. Asheet according to claim 14 wherein the material is PTFE.
 17. In anapparatus for producing a sheet of material for use in semiconductorprocessing, a stop for positioning between first and second pressingelements to limit the extent to which the pressing elements approach oneanother during a pressing operation, the stop comprising a reinforcingsection of a first thickness and a stop section of a second thicknesswhich is less than the first thickness; the reinforcing sectioncomprising a closed geometric shape which bounds a central portion ofthe stop; and the stop portion extending inwardly into the centralportion from the reinforcing portion, the stop portion spanning lessthan the entire area of the central portion to provide a void in thecentral portion.
 18. A stop according to claim 17 wherein the stopportion comprises a plurality of projecting fingers extending inwardlyinto the central portion.
 19. A stop according to claim 17 wherein thestop portion is about twenty-thousandths of an inch thick.
 20. A stopaccording to claim 18 wherein the fingers comprise triangular teethextending radially inwardly into the central portion from the annulus.21. A stop according to claim 18 wherein each of the fingers comprises atooth which is triangular in shape with an apex positioned inwardly of abase, the tooth having first and second sides intersecting one anotherat the apex, the first and second sides defining an acute angletherebetween.
 22. A stop according to claim 21 wherein the acute angleis about thirty degrees.
 23. A method of forming a layer of a materialon a substrate assembly comprising: heating the material; pressing thematerial between first and second flat pressing surfaces; disposing astop between the first and second pressing surfaces to limit the extentto which the first and second pressing surfaces approach one anotherduring pressing to thereby form a layer of a substantially uniformthickness having first and second major surfaces with the first andsecond major surfaces being formed by the flat pressing surfaces; andapplying one of the first and second major surfaces to a surface of asubstrate assembly.
 24. The method of claim 23 wherein the heating actcomprises heating the material until the material transitions to aplastic state without melting the material.
 25. The method of claim 23wherein the substrate assembly has an epoxy layer and the applying actcomprises applying one of the first and second major surfaces of thelayer to a surface of the epoxy layer of the substrate assembly.
 26. Amethod of forming a layer of material for use in forming a flat on asemiconductor, the material having first and second major surfaces whichare substantially parallel to one another and substantially flat, themethod comprising: pressing first and second optical flats toward oneanother; disposing a shim at least partially between the optical flatsto limit the extent to which the optical flats approach one another andto thereby establish the uniform thickness of the layer; providing anopen central region in the shim for receiving the material to be formedonto the layer; and flowing excess material outwardly from the centralregion into pockets provided in the shim during pressing.
 27. The methodof claim 26 further comprising heating the layer.
 28. The method ofclaim 26 wherein the material is FEP and comprising heating the layer toa temperature at which the layer becomes plastic but remains below themelting point temperature.
 29. The method of claim 26 wherein thematerial is PTFE and comprising heating the layer to a temperature atwhich the layer becomes plastic but remains below the melting pointtemperature.
 30. The method of claim 26 further comprising removingexcess material from the peripheries of the first and second pressingsurfaces and thereafter allowing the formed layer to cool.
 31. Asemiconductor substrate assembly in production comprising: asemiconductor flat comprised of semiconductor material and having afirst surface with at least one surface feature formed therein; an epoxylayer covering the first surface and the surface feature, the epoxylayer having a covering material engaging surface spaced from the firstsurface; a cover layer overlying the covering material engaging surfaceof the epoxy layer, the cover layer having first and second majoropposed surfaces with a thickness between the first and second majoropposed surfaces which is within at least five millionths of an inch ofbeing uniform, the first and second major opposed surfaces being withinone hundred angstroms of being flat.
 32. The semiconductor substrateassembly of claim 31 wherein the cover layer is formed of FEP.
 33. Thesemiconductor substrate assembly of claim 31 wherein the cover layer isformed from PTFE.
 34. The semiconductor substrate assembly of claim 31wherein the substantially uniform thickness of the cover layer is abouttwenty-thousandth of an inch.
 35. The semiconductor substrate assemblyof claim 31 wherein the first and second major surfaces are within fiftyangstroms of being flat.
 36. The semiconductor substrate assembly ofclaim 31 wherein the cover layer is transparent to ultravioletradiation.
 37. A method of planarizing a semiconductor wafer comprising:applying one major working surface of a sheet of material having firstand second opposed major working surfaces to an epoxy layer of asubstrate assembly, the first and second major working surfaces eachbeing within fifty angstroms of being flat and being at least withinfive millionths of an inch of being parallel to one another; pressing anoptical flat against the sheet of material and substrate assembly toplanarize the epoxy layer; and removing the sheet of material.
 38. Amethod according to claim 37 including the step of curing the epoxylayer through the sheet of material.